1. Field of the Invention
This invention relates to an Improvement of an apparatus for a head up display for the use In a motor vehicle, in which image information, such as characters or marks is presented forwardly of a front window or front glass of a motor vehicle in an overlapped relation to the foreground.
2. Prior Art
Heretofore, there is known an apparatus for a head up display for the use in a motor vehicle, in which image information such as characters or marks is presented forwardly of a front window or front glass of a motor vehicle in an overlapped relation to the foreground. A construction as shown in FIG. 1, for example, is known for this type of an apparatus for a head up display.
In FIG. 1, reference numeral 1 denotes an instrument panel; and 2, a front glass of a motor vehicle, respectively. A projection means 3 is disposed inside the instrument panel 1. This projector 3 includes a meter display 4 for displaying a vehicle information for a driver. This meter display 4 comprises, among others, a bright fluorescent character display tube (VFD) and a liquid crystal display (LCD), for example. A combiner 5 is formed on the front glass 2 by coating. The combiner 5 is a semitransparent reflective face having a reflective Index of 25% to 50%, for example. The projector 3 further includes a plane mirror serving as a middle mirror 6. The image information (vehicle speed, etc.) such as characters or marks displayed in the meter display 4 is reflected by the plane mirror 6 and introduced to the combiner 5 through a transparent cover 7 adapted to prevent dust. This image information is then reflected by the combiner 5 in a direction where a pair of eyes of a person riding on the motor vehicle (for example, driver) are present. The person driving in the motor vehicle can visually recognize the image information as if the image information (as represented by reference numeral 8) were present forwardly of the driver's seat, i.e., forwardly of the front glass 2. In other words, a virtual image 8 of the image information is formed forwardly of the front glass 2.
In the case where the front glass 2 is a planar glass as shown in FIG. 2, the image information. 9, even if reflected by this front glass 2, is not distorted (that is, the image information is visually recognized in a non-distorted condition) in accordance with the principles of reflection. In FIG. 2, reference numeral 10 denotes a left eye of the person riding in the motor vehicle; 11, a right eye of the person riding in the motor vehicle; and 9', a virtual image corresponding to the image information 9, respectively.
Actually, however, the front glass 2 has a configuration which is curved in a three dimensional direction and functions as a concave mirror (see FIGS. 3 and 4). Opposite side portions of the front glass 2 in the sense of a lateral direction are proportionally increased in curvature toward opposite side edges thereof. Similarly, opposite side portions of the front glass 2 in the sense of a vertical direction are proportionally increased in curvature toward upper and lower edges thereof. On the other hand, a central portion of the front glass 2 is generally constant in curvature. In the case where the front glass 2 is curved like this, let us presume here that a luminous flux P1 emitted from one point Q on the image information 9 reflected by the front glass 2 at a reflection portion R1 thereof and made incident to the left eye 10. At this time, a luminous flux P2 emitted from the one point Q is reflected by the front glass 2 and made incident to the right eye 11 which is located at the same level of height as the left eye 10. A reflection portion on the front glass 2 for this luminous flux P2 is represented by R2. The curvature of the front glass 2 is larger proportionally increased toward the edges of the front glass. Accordingly, the reflection portion R2 is height-wise displaced relative to the reflection portion R1. In FIG. 4, reference character H denotes an amount of the height-wise displacement between the reflection portion R1 and the reflection portion R2. Since the distortion (magnification) of the image information when looked through the left eye 10 which is located on the side where the curvature of the front glass 2 is small, is negligible, If any, because the front glass 2 can be regarded almost as a plane. However, the distortion (magnification) of the image information when looked through the right eye 11 which is located on the side where the curvature of the front glass 2 is large, is rather increased because the front glass 2 strongly exhibits its function as a concave mirror.
As a result, as shown schematically in FIG. 5, a virtual image originating from the image information 9 is visually recognized as represented by reference numeral 9a on the side of the left eye 10, and as represented by reference numeral 9b on the side of the right eye 11. In other words, the person riding in the motor vehicle looks at two overlapping virtual images (double image) 9a and 9b which are vertically and horizontally displaced and distorted. In this case, the horizontal displacement of the virtual images 9a and 9b can be corrected by the function of the brain of the person looking the virtual images. However, the vertical displacement of the virtual images 9a and 9b is impossible to be corrected by the function of the brain. As a result, the person riding in the motor vehicle tends to have such unpleasant feeling as eye fatigue ready to occur, uneasy feel or the like.
In order to correct the height-wise positional displacement of the virtual images 9a and 9b, there can be contemplated the use of a spherical concave mirror 12 as shown in FIGS. 6 and 7. However, if the concave mirror 12 is employed, there is a possibility that the imaging place is proportionally greatly displaced as it goes away from an optical axis O. That is, the concave mirror 12 generally has such an optical characteristic as aberration. Specifically, as shown in FIG. 6, parallel rays S1 near the optical axis O are converged to a place near a focal point f. On the other hand, parallel rays S2 remote from the optical axis S2 are converged to a place away from the focal point f. Accordingly, although the height-wise positional displacement of the image can be removed by the concave mirror 12, the distortion of the image cannot be removed. In FIG. 6, reference character C denotes a center of the curvature of the concave mirror 12, and in FIG. 7, reference numeral 13 denotes an object and 14, a virtual image of the object 13 produced by the function of the concave mirror 12.
It is also contemplated to remove the height-wise positional displacement of the double image using a convex mirror (for example, Japanese Patent Application No. Hei 2-167507; Filing date: Jun. 26, 1990; Title of the Invention: apparatus for a head up display (Japanese patent laid-open No. Hei 4-56633)). However, this apparatus has the shortcoming that the image information cannot be presented in a remote place due to optical characteristic of its convex lens.